a) Field of the Invention
The present invention concerns ionic compounds, their process of preparation and their use as photoinitiators for the cationic polymerization or cross-linking of monomers and prepolymers, or for the modification of solubility parameters of certain polymers which may be used as photoresists.
b) Description of Prior Art
A polymerization which involves a mechanism of the cationic type has many advantages. In particular, it is fast, even at low temperature, the rate of utiliation of the monomer is high and sensitivity towards atmospheric contaminants such as oxygen is low as compared to free radical or anionic polymerizations.
Monomers, prepolymers and polymers containing cycloaliphatic epoxy functions and their vinyl ethers are increasingly used such as in the paint, varnish, inks, glue and anti-adhesive support industries. Moreover, vinyl ethers generally appear free of toxicity contrary to acrylates or methacrylates. Monomers and prepolymers of the epoxy type or of the vinyl ether type may be polymerized by different methods, cationic polymerization being particularly interesting.
Cationic polymerization catalysts are generally considered as acids within the meaning of Bronsted HX (proton donors), or as acids within the meaning of Lewis (receptors of electronic doublets), these operating in the presence of a co-catalyst which is a source of protons. These acids must be sufficiently strong to ensure stability to the cationic species which is either carried by the monomer or by the growing macromolecular chain, which means that the corresponding anion X.sup.- should possess a nucleophilic power which is as low as possible. The Bronsted acids which are most often used as catalysts for cationic polymerization are CF.sub.3 SO.sub.3 H, HClO.sub.4, HBF.sub.4, HPF.sub.6, HAsF.sub.6 and SbF.sub.6. These acids are classified as follows with respect to initiation and propagation speeds, as well as obtention of the highest molecular weights: EQU CF.sub.3 SO.sub.3 H&lt;HClO.sub.4 .apprxeq.HBF.sub.4 &lt;HPF.sub.6 .apprxeq.HAsF.sub.6 .apprxeq.SbF.sub.6
More recently, compounds with acid character such as bis(perfluoroalkylsulfonyl)-imide (U.S. Pat. No. 4,031,036, Koshar et al) or bis(perfluoroalkylsulfonyl)methane (U.S. Pat. No. 3,632,843, Allen et al) have also been used.
It is known that the preparation in situ of polymerization catalysts has many advantages. The production in situ of an acid which is capable of catalyzing the cross-linking of a monomer enables indeed to prepare a fluid monomer or a prepolymer (thermoplastic or solution) and to give it its final properties, for example by a simple treatment with a radiation. This technique is very much used for inks, paints, adhesive films and anti-adhesive films. It should also be noted that the preparation of the acid in situ from a salt enables in many cases to prevent the storing and handling of acid compounds which are more corrosive than the corresponding salts.
The catalysts may be prepared in situ by heat treatment. For example, ammonium or metal salts of bis(perfluoroalkylsulfonyl)imide (U.S. Pat. No. 4,031,036, Koshar et al) or ammonium or amine salts of bis(perfluoroalkylsulfonyl)methane (U.S. Pat. No. 3,632,843, Allen et al) have been used to obtain in situ, by heating, the corresponding bis(perfluoroalkylsulfonyl)-imide or bis(perfluoroalkylsulfonyl)methane, which thereafter acts as catalyst. These catalysts, so called "latent", however, only present a limited interest due to the necessity of an extended heating at high temperature to obtain a release of the acid, this release being in addition progressive and not integral with the initiation. On the one hand, the result is a low reaction speed and on the other hand, polymers of mediocre quality with respect to molecular weight, polydispersity and coloring.
The acid catalysts may also be prepared in situ by actinic radiation (such as photons whose wavelength corresponds to ultraviolet, visible, .gamma. and X radiation) or by .beta.-radiation (beam electrons) on a suitable salt. Such a salt which is chemically labile under actinic or .beta.-radiation bringing about the release of the corresponding acid with a strong catalytic activity, is a photoinitiator. The advantages of such a process are numerous: the release of the catalyst by radiation is rapid and practically complete, which causes a simultaneous initiation of the growth of the chains, and therefore a more homogeneous distribution of the masses with a lesser polydispersity, and better mechanical properties. The polymerization may be carried out at a relatively low temperature which prevents decomposition or coloring of materials obtained, as well as the formation of bubbles when a solvent is used or when the reaction mixture contains a volatile additive which is intended to be maintained in the final material and which plays the role of plasticizing agent
U.S. Pat. No. 5,554,664 describes salts which can be activated under the effect of energy in which the cation comprises at least one cation selected from organometallic cations comprising an aromatic compound based on an arene or a pentadienyl ligand and a transition metal and also from iodonium, sulfonium, phosphonium and carbonium cations comprising a transition metal and in which the number of anions is sufficient to neutralize the charge of the cation, the anion being a tris(alkylsulfonylmethylide), a bis(alkylsulfonylimide), a tris(arylsulfonylmethylide) or a bis(arylsulfonylimide) salt in which the alkyl or aryl group is possibly perfluorinated or highly fluorinated. Salts of a simple metal, diazonium salts and ammonium salts are excluded. These salts may be used as photoinitiators for the cationic polymerization of olefins.
It is also known to use acids produced by actinic radiation to degrade the resins contained in a film constituting a photoresist. This technique is particularly efficient for photoresists with chemical amplification, in which a very small quantity of protons catalyzes the decomposition of groups such as esters containing a group derived from a tertiary alcohol (such as, for example, a tertiobutyl group), which is part of a macromolecular chain. This technique thus enables to modify the solubility parameters of a resin exposed to actinic radiation and to carry out operations of selective masking and engraving such as used in microelectronics.